The present invention is directed to a liquid crystal matrix display device which uses thin film transistors (hereinafter referred to as TFTs) as address devices for switching picture elements. More particularly the present invention is directed to a liquid crystal active matrix display device which uses, as address devices, reverse stagger type TFTs whose semiconducting film is made of amorphous silicon (a--Si).
FIGS. 3(A) and 3(B) show the construction of the conventional liquid crystal active matrix display device using reverse stagger type TFTs as address devices. This liquid crystal display device comprises a pair of cell substrates which are opposed to each other. The liquid crystal element is sealed in the gap between the pair of cell substrates, thereby forming a liquid crystal cell. One of the two cell substrates comprises gate electrodes 102, gate insulating film 105, a--Si (amorphous silicon) semiconducting film 106, insulating film 107, n.sup.+ --a--Si (n.sup.+ -amorphous silicon) contact film 108 for ohmic contact, source and drain electrodes 109, picture element electrodes 110 for display and protective film 111 laminated on an insulating substrate 101. The picture element electrodes 110 are arranged in a matrix-type fashion. An additional capacity Cs, parallel to the liquid crystal capacity, is formed to improve the picture element potential for retaining characteristics and minimizing the picture element electrode potential level shift at the time of the fall of the gate voltage, attributable to the overlapping capacity between the gate electrode 102 and the drain electrode 109. The additional capacity Cs is obtained by forming the transparent conducting film in two layers on the insulating substrate 101 with the extension of the gate insulating film 105 between the two layers being the means for isolating them for each other. The first layer of the conducting film serves as a ground electrode 112 and the second layer serves as the picture element electrode 110, as shown in FIG. 3(B).
According to the above conventional art, as mentioned above, the ground electrode 112 for providing the additional capacity Cs is made of a transparent conducting film. Therefore, the resistance of the ground electrode 112 for additional capacity Cs is high, and therefore causes a large load on the driver circuit. The resistance of the ground electrode 112 for additional capacity Cs could be reduced by increasing the thickness or width of the ground electrode film 112. However, a greater film thickness would result in a greater difference in level between the ground electrode 112 and the insulating substrate 101. This would thereby cause breakage of the source electrode and other elements of each TFT to be formed in a later process. On the other hand, a greater electrode width would result in a higher probability of short circuit occurring the ground electrode 112 for additional capacity Cs. It would further increase the probability the picture element electrode 110 and for short circuit between the ground electrode 112 and the source electrode 109. Finally, it would also result in a larger capacity occurring between the ground electrode 112 for additional capacity Cs, and the source electrode 109. This would thus cause a level shift in signals.
In addition, the thicker or wider that the ground electrode 112 for additional capacity Cs is, the larger number of manufacturing processes necessary. This would not be favorable.